One of the longest-running controversies in evolutionary biology has been: What was the oldest branch of the animal family tree? For the best part of a century, sponges were held to be the earliest form of animal life, based primarily on the observed simplicity of their structures. However, a new kind of genetic analysis reveals that comb jellies are older than sea sponges.
Previous genetic analyses used in phylogenomic studies were based on a collection of a large amount of data, building a set of relationships between them and then built a plausible family tree. This method was successful in 95 percent of the cases. Remaining 5 percent were the subject of debates among scientists in last decade, which eventually lead to the development of a new analyzing technique.
The study, led by Antonis Rokas of Vanderbilt University in Tennessee, analyzed hundreds of thousands of genes sequenced from target organisms to try to resolve heredity issues that up until now have seemed intractable. These targeted organisms were animals, plants, and fungi. In these analyses, we only use genes that are shared across all organisms. The trick is to examine the gene sequences from different organisms to figure out who they identify as their closest relatives, explains Rokas.
The results of this analysis found a so-called “phylogenetic signal” that favored comb jellies over a sponge. The results have shown significantly more genes to support their “first to diverge” status than sea sponges do.
Comb Jellies Facts
Comb jellies are ancient animals and voracious carnivores that belong to the phylum Ctenophora. This includes around 100 species.
- Their radially symmetric body contains 8 rows of comb-like plates of fused cilia that beat in order for them to move through the water.
- The comb rows diffract light to create a rainbow display as they swim. Some comb jellies, additionally, exhibit bioluminescence or the emission of light by chemical reactions.
- About half of the comb jelly species feed by the use of a pair of tentacles that do not sting but instead contain sticky cells called colloblasts. Zooplankton will stick to their tentacles and they will draw the prey into their mouth.
- Some comb jellies have no tentacles. Instead, their mouths contain hooks to bite off chunks of prey and secrete a poison to paralyze prey.
- They possess a gravity sensitive structure called a statocyst that gives them a sense of up and down in the water.
- Comb Jellies have no brain but contain a loose network of nerves called a nerve net.
- They are hermaphrodites. Eggs and sperm are cast into the water and fertilization take place. Their larvae grow rapidly as their entire lifespan is only around a few months.
Ctenophores typically prefer warmer waters and tropical areas near the surface in both shallow and deep habitats. Overfishing has caused an increase in the population of ctenophores because of less predation.